Conventionally, as materials showing a positive PTCR, there have been proposed compositions in which various semiconductive dopants are added to BaTiO3. Since these compositions have the Curie temperatures around 120° C., these composition need to be shifted in the Curie temperatures depending on the applications.
For example, although it has been proposed to shift Curie temperature by adding SrTiO3 to BaTiO3, in this case, the Curie temperature is shifted only in a negative direction and is not shifted in a positive direction. Only PbTiO3 is known as an additive element for shifting Curie temperature in a positive direction. However, PbTiO3 includes an element for bringing about environmental pollution, and therefore a material which does not use PbTiO3 has been desired in recent years.
In a BaTiO3 semiconductor porcelain, with an object of preventing a reduction in a temperature coefficient of resistivity by Pb substitution, as well as reducing a voltage dependency and promoting a productivity and a reliability, there has been proposed a method of producing a BaTiO3 semiconductor porcelain in which a composition obtained by adding one or more kinds of any of Nb, Ta and a rare earth element to a composition in which x in Ba1−2x (BiNa)xTiO3 where a portion of Ba of BaTiO3 is substituted by Bi—Na is controlled to a range of 0≦x≦0.15, is sintered in nitrogen, and thereafter subjected to a heat treatment in an oxidizing atmosphere.
Patent Reference 1: JP-A-56-169301